Fix speed regression

With this [commit](https://github.com/jesscss/jess/commit/924ed7962c360a3409dd3bb283f4d9befe31340d), #31 was fixed, but it changed the benchmark speed from ~1.4s to ~1.7s. An enhancement here would be to figure out if there's any way to speed up any of these structures. Is Set slow? Is it walking nodes that's slower?